Carbon paper

ABSTRACT

AN ELECTRICALLY CONDUCTIVE, CHEMICALLY INERT, POROUS STRUCTURALLY COHERENT WEB OF CARBON FIBRES WHICH HAS A CARBON COATING THEREON. THE CARBON COATINGS ON THE RESPECTIVE CARBON FIBRES ARE INTERGROWN WITH ONE ANOTHER AT THOSE LOCATIONS WHERE THE COATED FIBRES CONTACT ONE ANOTHER. AT LEAST ABOUT FIFTY PERCENT OF THE VOLUME IN THE WEB IS VOID OF COATED FIBRES.

Aug. 13, 1974 THOMAS T. OMORI ET AL GARBON PAPER '1104 July 3. 1972United States Patent O 3,829,327 CARBON PAPER Thomas T. Omori, Glendale,and Hiroshi Imaoka, Gardena, Calif., nassignors to Kreha Corporation ofAmerica, Gardena, Calif.

Filed July 3, 1972, Ser. No. 268,600 Int. Cl. C01b 31/04; HOIm 13/02U.s. ci. 117-226 12 caims ABSTRACT OF THE DISCLOSURE The presentinvention relates to a pure carbon web that is electrically conductive,chemically inert, porous, and structurally coherent. These webs areparticularly suited for use as electrodes in fuel cells and other harshenvironments where electrical conductivity is required of a thin poroussheet of material that must be thermally stable and chemically inerteven at elevated temperatures of several hundred degrees Fahrenheit.Also, the web is particularly suited for use where it must retain itsphysical, chemical, and electrical characteristics and dimensions whileit is subjected to handling, transportation, and shaping prior to itsbeing assembled into a completed working device.

Previously, porous webs that were intended for use in extremeenvironments that demanded chemical, thermal, and structural stabilitywere deficient or defective in achieving these characteristics. Thematerials from which it was proposed to manufacture such porous webswere difficult and expensive to product and manufacture. In general, theproposed Structures vwere unsatisfactory for the intended purposes.

According to the present inventior, a web of carbon fibres having acarbon coatingfis prepared so that the individual carbon fibres aresubstantially uniformly coated With an annular coating or jacket .whichis composed of pure carbon. The carbon coatings are intergrown with oneanother at the intersection between the carbon fibres so that the carboncoatings serve to bind or interlock the carbon fibres into an integralstructurally coherent web. The interlocking of the carbon coatings atthe locations where the coated fibres contact one another provides goodelectrical connections throughout the entire mass of the web as well asproviding structural integrity for the'web. The pure carbon web ischemically inert even in acidic or basic environments at elevatedtemperatures.

The carbon coating is applied by known chemical vapor deposition methodsto a carbon fibre web. The chemical vapor deposition procedure iscarried on for a period of time sufficient to substantially uniformlycoat the carbon fibres with the pyrolytic carbonaceous material and toprovide interlocking between the coated fibres. The chemical vapordeposition procedure is terminated before the deposits grow to theextent that more than 50 percent of the Volume in the web is occupied bythe coated fibres. Preferably, the chemical vapor deposition procedureis terminated when the carbon deposits have grown to such an extent thatbetween 30 and 50 percent of the volume in the web is occupied by thecoated fibres.

According to one method, the carbon fibre web or paper, on which thecarbon coating is deposited by chemical vapor deposition procedures, isprepared from carbon fibres using the following procedure. Carbon fibreswhich may be composed of amorphous carbon, graphite, or ad- 3,829,327Patented Aug. 13, 1974 ice mixtures of these forms of carbon are choppedinto finite lengths. Conveniently, the basic carbon fibre is choppedinto lengths of approximately one-eighth of one inch. The chopped fibresare prepared into a mat or paper on a paper-making machine. The carbonfibres by themselves are generally very hard and smooth so that they donot adhere readily to one another. In order to promote formation of acarbon paper, a binder, such as polyvinyl alcohol, is admixed with thefibre as it is being processed into paper. The polyvinyl alcohol binderserves to bind the carbon fibres together so that the fibres will clingtogether to form a structurally coherent web. The carbon fibre paper isthen impregnated with a dilute solution of a charring synthetic resin,such as, for example, a phenolic resin. The phenolic-impregnated,polyvinyl alcohol bound carbon fibre web is then subjected to acarbonizing step at, for example, approximately 1000 degrees centigradeto volatilize the polyvinyl alcohol binding and to convert the phenolicresin to carbon. The resultant carbon fibre web is bound together bycarbon bridges between the carbon fibres. These carbon bridges are theresidue that is left from the charring of the dilute phenolic resin. Thecarbon paper or web is now a pure carbon product, existing almostentirely in the amorphous carbon form. The polyvinyl alcohol binding hasbeen replaced by carbon bridges formed from the residue of the charringsynthetic resin. The resultant carbon bound, carbon fibre mat is thensubjected to chemical vapor deposition according to known procedures.

According to one known procedure, chemical vapor deposition isaccomplshed by subjecting the carbon bound, carbon fibre web totemperatures between about 1300 degrees centigrade and 2800 degreescentigrade or more at a reduced pressure in the presence of a dilutehydrocarbon gas. The hydrocarbon gas decomposes to a non-sooting carbonvapor that deposits a carbon coating on the heated, carbon bound, carbonfibre web in a very uniform coating. At the higher temperature thecarbon web is almost entirely in the graphite form. As the temperaturedecreases, the amorphous carbon form increases, and at the lowertemperatures the carbon web is almost entirely in the amorphous carbonform. The resultant porous carbon web has a very uniform porosity. Themechanical and electrical characteristics of the web are very uniformwhen measured along different parallel axis in the web. The mechanicaland electrical characteristics vary somewhat with the orientation of themajority of the fibres.

The temperatures employed during the preparation of the carbon fibre webwith the carbon coating determine whether the resulting structure willbe composed of carbon in the graphite form or the amorphous carbon form.The term carbon is used herein to describe the material in the fibresand in the coatings in both the amorphous carbon and graphite forms inwhich a majority of the coating is graphite and a minority is carbon. Asused herein the term carbon is intended to include all forms of carbonand all admixtures of these several forms.

The electrical properties of the electrically conductive porous web canbe tailored so that they are different when measured in differentdirections in the plane of the web. In general, the electricalresisitivity decreases in a particite form increases. The electricalresistvity, at least in one'direction, is less than about 0.020 ohmcentimeter.

The carbon coatngs may be activated, if desired, for example, bytreating the finished porous web with Steam.

The web, if desired, may be manufactured from carbon filaments by aweaving process rather than a paper-makng process.

The chemical vapor deposition procedure results in the formation of acarbon coating which substantially uniformly covers each of theindividual carbon fibres in the carbon bound, carbon fibre mat. Each ofthe carbon fibres is encased in its own individual coating, and wherethe carbon fibres touch one another or are in close proximity to oneanother, the carbon coatings are grown together so that the fibres areinterlocked at their intersections by their respective carbon coatings.The binding formed by the charring synthetic resin is almost entirelyreplaced by the interlocking of the coatings. The electricalconductivity of the porous web is greatly increased by the presence ofthe interlocked carbon coatings. The intergrowth of the carbon coatingsat the intersections of the respective individual fibres provides a goodmechanical connection throughout the entire web so that electricityflows easily across the web without encountering any open connections.Also, the interlocking of the respective carbon coatings greatlyimproves the structural strength and integrity of the web. The area ofthe contact between the fibres due to the intergrowth of the coatings ispreferably at least approximately as great as the cross-sectional areaof the uncoated fibre measured normal to the longitudinal axis of thefibre.

The pure carbon porous webs are prepared with as great a degree ofporosity as possible. In general, more than 50 percent of the Volumewithin the web is void, and preferably from about 50 to 70 percent ofthe Volume in the web is empty void. The porosity promotes the flow offluids through the web when the web is being used. Also, the high degreeof porosity permits the fluid transmission properties of the web to betailored to a partcular set of fluid flow requirements by irnpregnatingthe web with an inert material, such as polytetrafluorethylene, until aparticular degree of desired porosity is achieved. Preferably, the coresize of the web is less than approximately 60 microns. The highpercentage of void space within the porous web also provides spacewithin which to incorporate a finely divided catalyst. In general, whena catalyst is incorporated into the porous web, it is bound to the webby some inert binder, such as polytetrafiuorethylene.

The physical dimensions of the completed pure carbon web may be variedwidely, as desired, within the size capacity of the equipment in whichit is manufactured. In general, the length and Width of the web mayrange from a few inches to several feet. The thickness of the web isgenerally not less than 0.2 millimeter nor more than 5 millimeters,although somewhat greater or lesser thicknesses may be utilized ifdesired. In general, the thickness of the web is less than about onemillimeter. The thickness of the web is generally substantially lessthan about the average length of the coated fibres and preferably lessthan about one-third of the average length of the coated fibres.Preferably, the bulk density of the carbon web is at least 0.25 gram percubic centimeter, and the thickness is at most 0.625 millimeter. Theweight of the carbon web generally ranges from about 140 to 220 gramsper square meter.

When the web is used where electrical current is taken from or appliedto the web, the connectiions to the web conveniently take the form of aconductive band of material extending across one edge of the web. Theconductive band may be oriented with respect to the orientation of themajority of the fibres to take advantage of any directionalcharacteristics that the web may possess. For example, if the lowestresistvity is desired, the conductive band should be positioned so thatit extends generally perpendicular to the direction in which themajority of the fibres are oriented.

In the drawings there is illustrated:

FIG. 1, a plan view photograph taken at a magnification of approximately200 times of a carbon bound, carbon fibre web prior to chemical vapordeposition;

FIG. 2, a view of the completed pure carbon web similar to FIG. l takenat a magnification of approximately 200 times after the carbon web hasbeen subjected to chemical vapor deposition;

FIG. 3, an edge view of a carbon bound, carbon fibre web prior tochemical vapor deposition taken at a magnification of approximately 200times; and

FIG. 4, a view of the completed pure carbon web similar to FIG. 3showing the carbon web after the chemical vapor deposition procedure hasbeen completed taken at a magnification of approximately 200 times.

Comparison of the web before and after chemical vapor deposition clearlyreveals that the chemical vapor deposition results in the application ofa very uniform coating on the individual carbon fibres. Also, the carbonbinding illustrated particularly by the thin sheets extending betweenfibres in FIG. 1 is removed during the chemical vapor depositionprocedure and is replaced by the intergrowth and nterlocking of thecarbon coatings on the respective fibres. The carbon coatings areparticularly apparent in FIG. 4.

The web shown in FIGS. l and 3 was prepared from carbon fibres that Werechopped to As-inch lengths and manufactured into a mat on a paper-makingmachine, using approximately 10 percent polyvinyl alcohol to bind themat together. The resultant polyvinyl alcohol-carbon fibre mat hadapproximately 70 percent of its fibres oriented in one direction. Thepolyvinyl alcohol bound carbon fibre mat was then impregnated with adilute solution of charring phenolic resin and was carbonized at atemperature of about 1000 degrees centigrade. The polyvinyl alcohol wascompletely volatilized at this temperature, leaving the carbon fibresbound together by carbon bridges produced from the charring of thephenolic resin. These carbon bridges are evident in FIG. 1. Theelectrical resistivity of this carbon bound, carbon fibre web was about0.17 ohm centimeter measured in the direction in which the majority ofthe fibres are oriented. The weight per area of this material was about37 grams per square meter. Its density was about 0.13 gram per cubiccentirneter, and its thickness was about 0.26 millimeter. This materialwas subjected to chemical vapor deposition at about 1300 degreescentigrade for a period of time suicient to deposit a carbon coatinghaving a thickness such that the diameter of the coated fibres isapproximately twice that of the uncoated fibres. The electricalresistvity of the pure carbon porous web measured in the direction ofthe majority of the fibers is about 0.015 ohm centimeter; the thicknessis about 0.5 millimeter; the weight per area is about 180 grams persquare meter; and about 65 percent of the Volume within the web is void.The carbon in this web is substantially all in the amorphous carbonform.

The chemically inert porous webs according to this invention aregenerally applicable to all categories of extreme lquid and gaseousfiuid environments even at elevated temperatures.

What is claimed is:

1. A porous web elect'ode comprising:

a plurality of carbon fibres;

a carbon coating substantially uniformly covering each of said carbonfibres, the coated fibres being combined together to define said porousweb, a substantial portion of said coated fibres being positioned inapproximately the plane of said porous web, the said carbon coatings onsaid coated fibres generally being intergrown at locations where saidcoated fibres contact one another, at least about fifty percent of theVolume in said porous web being void of coated fibres, said porous webhaving a weight of from about to 220 grams per square meter and aspecific resistivity in at least one direction of less than about 0.020ohm centimeter; and

means for making electrical connections to said porous web.

2. A porous web of claim 1 wherein the carbon fibres contain asubstantial amount of graphite.

3. A porous web of claim 1 wherein the thckness of said web is less thanabout the average length of the coated fibres.

4. A porous web of claim 1 Wherein the thickness of said web is lessthan about one millimeter.

5. A porous web of claim 1 wherein the carbon coatings are intergrownwith one another at the intersections of the carbon fibres to such anextent that the area of the contact between the coated fibres at thesaid intersections is approximately at least as great as thecross-sectional area of the said carbon fibres.

6. A porous web of clairn 1 wheren the carbon coatings on the coatedfibres are substantially pure graphite.

7. A porous web electrode of claim 1 wherein at least about sixtypercent of the coated fibres are oriented in one general direction andthe means for making electrical connections to said porous web includesa conductive band extending generally perpendicular to said one generaldirection.

8. An electrically conductive porous web comprising:

a plurality of carbon fibres;

a carbon coating substantially uniformly coverng each of said carbonfibres, the coated fibres being combined together to define said porousweb, a substantial portion of said coated fibres being positioned inapproximately the plane of said porous web, the said carbon coatings onsaid coated fibres generally being intergrown with one another at theintersections of the carbon fibres in said porous web to such an extentthat the area of the contact between the coated fibres at saidintersections is approximately at least as great as the cross-sectionalarea of said carbon fibres, from about fifty to seventy percent of thevolume in said porous web being void of coated fibres, said porous webhaving a thickness from about 0.2 to 1 millimeter, said porous webhaving a Weight of from about 140 to 220 grams per square meter and aspecific resistivity in at least one direction of less than about 0.020ohm centimeter.

9. An electrically conductive porous web comprising:

a plurality of carbon fibres;

a carbon coating substantially uniformly covering each of said carbonfibres, the coated fibres being combined together to define said porousweb, a substantial portion of said coated fibres being positioned inapproximately the plane of said porous web, the thickness of said porousweb being from about 0.2 to 5 millimeters, said thckness being less thanabout onethird the average length of said carbon fibres, the said carboncoatings on said coated fibres being generally intergrown at locationswhere said coated fibres contact one another, at least about fiftypercent of the Volume n said porous web being void of coated fibres, thepore size of void space within said porous web being less thanapproximately 60 microns, said porous web having a weight of from aboutto 220 grams per Square meter and a specific resistivity in at least onedirection of less than about 0.020 ohm centimeter.

10. A porous web of claim 9 wherein the carbon coating comprisesactvated carbon.

11. A process of manufacturing a porous web electrode comprising:

subjecting a web of carbon fibres to chemical vapor deposition ofcarbon;

continuing said chemical vapor deposition of carbon until asubstantially uniform carbon coating is formed on said carbon fibres andthe resultant coated fibres are generally intergrown at locations wheresaid coated fibres contact one another;

discontinuing said chemical vapor deposition of carbon before the coatedfibres occupy more than about fifty percent of the Volume in said web,and after said porous web reaches a Weight of from about 140 to 220grams per square meter and a specific resistivity in at least onedirection of less than about 0.020 ohm centimeter; and

recovering the resultant electrically conductve, chemically inert,structurally coherent, porous web electrode.

12. A process of claim 11 including selecting a web of carbon fibresthat has a substantial portion of the carbon fibres positioned inapproximately the plane of said web, and a majority of said carbonfibres are oriented in one general direction.

References Cited UNITED STATES PATENTS 3,627,571 12/1971 Cass 117-2283,619,286 11/1971 Gutnajer 117-46 CG 3,527,564 9/ 1970 Moore 423-4473,471,3 83 10/ 1969 Tiedemann 204 294 3,369,920 2/1968 Bourdeau117-119.6 3,164,487 1/ 1965 Carley-Macauley 117-46 CG 3,32l,327 5/1967Blanchard 117-46 CG 3,725,110 4/ 1973 Rodgers 117-46 CG OTHER REFERENCESResearch Development on Advanced Graphite Materals, Tech. Rep. #WADD TR61-72, vol. XXVII (Air Force Materials Lab.), August 1969, pp. 43-61,Wright- Patterson Air Force Base, Ohio.

RALPH S. KENDALL, Primary Examiner M. F. ESPOSlTO, Assistant ExaminerU.S. Cl. X.R.

117-46 CG, 106 R, 228; 136-120 FC, 121, 122; 204-290, 294; 423-447Disclaimer 3,829,327.-Th0mas T. Omo'm', Glendale, and H z''oshi lmaolca,Gardena, Calif. CARBON PAPER. Patent dated Aug. 13, 1974:. Disclaimerfiled Mar. 27, 1978, by the assgnee, Kreha Oorpomton of America.

Hereby enters this dsclamer to clams l through 12, nclusive, of said.

patent.

[Oficial Gazette July 11, 1978.]

